Most stuff on hvac is controlled with 24vac. Some need more power to run say a fan motor or other internal stuff, but some (the one I have, for example) only need 24vac to run. One lead would be the common side of the 24vac transformer, and the other would be switched to turn it on when needed. This is often available on the thermostat leads on the control board. Often this could be the fan control, or if the control board has a humidifier lead contact, that may be all that is required. Normally, you don't run the humidifier unless the fan is on, so that's a possible connection.
Some use a separate current sensing switch (relay) on the line voltage lead to the fan to trigger the humidifer. This part is an accessory and may be available separately.
The symbology on hvac control diagrams is not the same as many electronic schematics, but you should be able to figure it out if you study it a bit.